This invention relates to packet-based data communication networks and particularly an Ethernet network or similar network wherein data may be transferred between devices, particularly a hub and another network station such as a personal computer, at at least two different rates, the devices being able to establish a selected data rate by means of auto-negotiation. The invention is particularly though not necessarily exclusively intended for use with an auto-negotiation scheme as defined in IEEE 802.3, 1998 Edition, Clause 28.
As switching and other technology has improved, the rate at which data may be transferred between network devices has increased. Currently there are various standards to which successive generations of network devices has been designed to operate. In particular a standard data rate of 10 Megabits per second has been partly supplanted by a standard of 100 Megabits per second and is expected to be supplanted by an even faster rate, 1 Gigabit per second. An increase in standard data rates may be expected in the future.
However, network devices operating according to obsolescent standards, normally at a lower data rate or rates, continue to be used. It therefore follows that it is known for a network hub to be capable of exchanging data with another network station at a plurality of different selectable rates. It is also known, and specified in for example the above mentioned network standard for a hub and a station to which it is connected to commence a xe2x80x98conversationxe2x80x99 by means of a process of auto-negotiation, wherein a device such as a network hub establishes by means of interrogation and response whether the other station is capable of transmitting and receiving data at the higher of two rates or a rate of which is highest of a multiplicity of rates, so that thereafter the devices, such as the hub and the other station, exchange or transmit information from one to the other at the selected higher or highest mutually acceptable rate. Auto-negotiation is fully explained in, for example, IEEE Std 802.3, 1998 Edition, published by the Institute of Electrical and Electronics Engineers, Inc., NY-10017-2394, USA.
It is known to maintain a connection between a hub and another station by means of a continuous sequence of symbols representing an idle state. When one station at either end of the connection wishes to send data then by convention it sends at least a first symbol followed by a second symbol, representing a xe2x80x98carrierxe2x80x99 signal. The data then follows in the format defined by the appropriate standard. At the end of the data packet a third predetermined symbol, followed by a fourth predetermined symbol, is sent so as to represent the end of the carrier. Such a sequence is conventionally repeated for each and every packet transmitted between the stations. Typically, the first and second symbols are a J and K symbol and the third and fourth symbols are a T and R symbol.
In our earlier patent application number 9810810.3, filed May 20, 1998, we describe and claim an improvement which includes the monitoring at one of the stations, namely the hub, the occurrence of an error represented by a symbol other than an idle state immediately followed by an idle state symbol, comparing a rate of occurrence of errors with a threshold and forcing the data connection to proceed at the lower rate if the aforementioned error rate exceeds the threshold.
Such a scheme is effective to monitor the quality of the connection. For example, although the devices which are connected by auto-negotiation may have the ability and the performance appropriate to a higher rate achieved by auto-negotiation, the physical link between them may not be of sufficient quality or may become degraded, so that the reliability and throughput of the connection is insufficient or is reduced. The earlier proposal facilitates the detection, by the monitoring of specific errors of a loss of quality of the connection or insufficient quality and if the error rate representing a parameter of the quality of the cable exceeds some selectable threshold rate, the link may be automatically downgraded to a lower rate selectable by auto-negotiation.
However, a system of that kind, and systems of possibly the same general character, may in a variety of circumstances fail to complete a link between devices properly.
For example, both ends of a link may be capable of higher rate operation yet the physical link may not be capable of supporting the selected higher date rate. For example, in the case of 10BASE-T/100BASE-TX operation, it is possible that certain channel characteristics can cause a situation wherein auto-negotiation can complete, because it uses the robust 10BASE-T link pulse signaling system, and select a highest common denominator of 100 Megabits per second yet the channel is not of sufficient quality for the 100BASE-TX link to be established.
10BASE-T link pulses are more likely to be correctly received due to the higher transmit amplitude level and margin at the receive threshold matching. Such a communications link is more reliable than a 100BASE-TX link. The 100BASE-TX linkOk requires a reliable presence of the received idle symbol stream and on a low quality link this may not be achieved.
When this occurs, the auto-negotiation state will time out as it never sees the required xe2x80x98link okxe2x80x99 and restarts auto-negotiation. In the particular example, the state machine which controls the auto-negotiation process (shown in FIG. 28-16 of the aforementioned publication) will not execute the transition from the state xe2x80x98FLP LINK GOOD CHECKxe2x80x99 to xe2x80x98FLP LINK GOODxe2x80x99 but will execute the transition to xe2x80x98TRANSMIT DISABLExe2x80x99 and thence to xe2x80x98ABILITY DETECTxe2x80x99, recommencing an auto-negotiation sequence. The new sequence of auto-negotiation again results in a selection of the higher rate (100BASE-TX) and owing to the sub-standard link the cycle will repeat continuously. Establishment of a link between the devices will not occur.
Another difficulty that exists arises from the provision of a substandard link with a device which is capable of xe2x80x98smart auto-sensingxe2x80x99 with a high quality receiver, connected to a far end device which has a low quality receiver and is not capable of xe2x80x98smartxe2x80x99 auto sensing. Here a xe2x80x98high-qualityxe2x80x99 receiver is meant to refer to a receiver which by virtue of a decision-feedback equalizer or otherwise has a high immunity to inter-symbol interference. Also the qualification xe2x80x98smartxe2x80x99 refers to a device of the general character described in the aforementioned application, that is to say a device which is capable of auto-negotiation, that is to say selection of the higher rate of or highest feasible rate of a multiplicity of possible data rates preferably in accordance with the aforementioned standard and is also capable of monitoring the link for the occurrences of errors, so as to be capable of downgrading the link to a lower of the possible rates in the event of detection of an undue proportion of specific errors. Notwithstanding the robustness of such a scheme, various situations can occur which may defeat the xe2x80x98smartxe2x80x99 device. The link could be established yet the non-smart device with the low quality receiver could be subject to multiple errors whereas the smart device with the high quality receiver would be immune to them. There is now in effect a substandard link that a xe2x80x98smartxe2x80x99 auto sensing scheme would not protect.
In this situation the continuous auto-negotiation would still occur because it would be forced by the device that was unable to establish the link.
The present invention is based on the detection of occurrences of an auto-negotiation process or cycle. This could be achieved in a variety of ways, depending on the nature of the auto-negotiation. One example is the use of bit 6.1 as defined in clause 28.2.4.1.5 and Table 28-5 of the aforementioned standard, this bit being a xe2x80x98Page-Receivedxe2x80x99 bit in an auto-negotiation expansion register which is one of the dedicated registers for a state machine controlling the auto-negotiation function. This bit is automatically set to logical xe2x80x981xe2x80x99 to indicate that a new Link Code Word has been received and stored in the auto-negotiation link partner ability register and is automatically set to logical zero when this register is read. Such a bit will therefore be set high each time the auto-negotiation function is executed by a physical layer device and may be used in conjunction with a threshold, defining for example a limit on the permitted number of executions of the auto-negotiation function, to force the auto-negotiation function to advertise a lower acceptable data rate. This modification can readily be included in an existing auto-negotiation algorithm as an additional condition.